The invention relates to blow molding a parison so as to form a portion thereof around a fixed pin.
In blow molding plastic articles, such as water tanks, it is sometimes necessary to form an inwardly extending, hollow boss for receiving a fastener, most typically a screw. For example, such a boss in a water tank can receive a screw therein for mounting to a wall bracket. The boss is conventionally formed by using a pin fixed to a mold part and extending from its mold face. In blow molding a parison, a portion of the parison forms around the pin to produce the desired hollow boss with a closed end. Considerable thinning of the parison wall at the boss end usually occurs during blow molding. As a result of the undesirably thin boss end, a screw as threadedly received in the hollow boss can sometimes penetrate and/or crack the boss end. This leads to leakage of water from the water tank.
It is, therefore, an object of the invention to provide a blow molding process and apparatus for use therein, which can produce a hollow boss with a thicker end to thereby assist in preventing penetration and/or cracking by a screw received in the boss.
The above object is realized with regard to an apparatus comprising: a mold part having a mold face and an opening extending through the mold part which is hereafter denoted as a mold opening, the mold opening having an end at the mold face; a sleeve having an end and slidably received in the mold opening; a pin having an end and extending through the sleeve, which thereby fits over and around the pin; a means for fixedly positioning the pin relative to the mold part so that the pin extends beyond the mold opening end with the pin end spaced therefrom outside of the mold opening; and a reciprocation means for selectively reciprocating the sleeve between an extended position and a retracted position, where in the extended position the sleeve end is substantially flush with the pin end and in the retracted position the pin extends beyond the sleeve end with the pin end spaced therefrom outside of the sleeve. The reciprocation means preferably employs application of fluidic pressure to move the sleeve between its extended and retracted positions. A pneumatic reciprocation means that uses air as the fluid is particularly preferred.
A blow molding process in accordance with the invention comprises: (a) providing (i) a mold part having a mold face and a mold opening, as described above, having an end at the mold face, (ii) a sleeve having an end and slidably received in the mold opening, and (iii) a pin having an end and fixedly positioned relative to the mold part so as to extend through the sleeve and beyond the mold opening end with the pin end spaced therefrom outside of the mold opening; (b) positioning the sleeve in an extended position with the sleeve end substantially flush with the pin end; (c) initiating blow molding of a molten parison with respect to the mold face and sleeve in its extended position; and (d) retracting the sleeve to a retracted position, in which the pin extends beyond the sleeve end with the pin end spaced therefrom outside of the sleeve, at a predetermined delay time after (c), at which time the parison is still sufficiently molten to allow a portion of the parison to form around that portion of the pin extending beyond the sleeve end.
It has been found that by blow molding the parison with the sleeve initially extended, followed by retraction of the sleeve after a predetermined delay time, a boss with a desirably thicker end results. This is clearly demonstrated in a subsequently described example.